Organic light-emitting devices are luminescent devices utilizing light that is emitted when excitons of fluorescent organic compounds move back to the ground state, the excitons being formed by sandwiching a thin film that contains the fluorescent compounds between an anode and a cathode and injecting electrons and holes (positive holes) from the respective electrodes.
A study by Eastman Kodak Company in 1987 (Appl. Phys. Lett. 51, 913 (1987)) reports that a light emission of about 1000 cd/m2 was observed under an applied voltage of about 10 V. In the study, the device had a function-separated type bilayer structure and used ITO as the anode, a magnesium-silver alloy as the cathode, an aluminum quinolinol complex as an electron-transporting material and a light-emitting material, and a triphenylamine derivative as a hole-transporting material. Related patent documents include U.S. Pat. No. 4,539,507, No. 4,720,432, and No. 4,885,211, etc.
Moreover, various light emissions in a range of from ultraviolet to infrared can be accomplished by using various different fluorescent organic compounds. Recently, a lot of studies on various compounds have been conducted actively. For instance, those studies are disclosed in U.S. Pat. No. 5,151,629, No. 5,409,783, No. 5,382,477, No. 5,130,603, No. 6,093,864, No. 5,227,252, Japanese Patent Applications Laid-Open No. 5-202356, No. 9-202878, and No. 9-227576.
In addition to the organic light-emitting devices using small molecular materials as described above, the organic light-emitting devices using conjugated polymers have been reported by the group of Cambridge University (Nature, 347, 539 (1990)). This report discloses that a film of polyphenylenevinylene (PPV) is formed by coating method and it is surely observed that a single layer of the film emitted light. Related patent documents on organic light-emitting devices using conjugated polymers include U.S. Pat. No. 5,247,190, No. 5,514,878, No. 5,672,678, No. 5,317,169, No. 5,726,457, and Japanese Patent Application Laid-Open No. 5-247460, etc.
Thus the remarkable progresses have been made in organic light-emitting devices, so that the recent luminescent devices are characterized by high brightness even when a lower voltage is applied, diversity in emission wavelengths, and rapid response, and thin and lightweight luminescent devices can be obtained, which suggests a wide range of possibilities of the device being utilized for various uses.
However, under the present circumstances, light emissions exhibiting higher brightness or higher conversion efficiency are required. Furthermore, there are still many problems in regard to durability such as change with elapse of time due to prolonged use and deterioration or the like, by moisture and the atmospheric gas containing oxygen. Moreover, in view of application to a full-color display etc., blue, green, and red light emissions having higher color purities are needed, and this problem has not been solved sufficiently yet.